1. Field of the Invention
The present invention relates to a current switch capable of precisely switching the current from a current source that uses a metal-oxide semiconductor (MOS) transistor or a metal-insulator semiconductor (MIS) transistor, and the phase-locked loop (PLL) or digital-to-analog (DA) converter that utilizes it.
2. Description of the Related Art
There are circuits that supply to capacitance and resistance, for example, current from a current source that uses an MOS transistor or an MIS transistor, either using the voltage of capacitance after a fixed period of time, or the voltage generated by resistance. For instance, the input voltage of a PLL's voltage-controlled oscillator requires high precision, and when generating input voltage by charging or discharging capacitance proportional to phase difference, current from a current source must be precisely supplied for charging/discharging purposes. Further, in a DA converter, high-precision analog voltage can be generated by precisely supplying to a resistance device current from a current source selected in accordance with the digital signal that is input.
FIG. 11 depicts a conventional current switch. This circuit example is a current switch that supplies to the load circuit comprising capacitance C.sub.O current I.sub.1 for charging via switch SW1, and current I.sub.2 for discharging via switch SW2. Therefore, it is useful as a current switch even when used solely as a charging current switch or solely as a discharge current switch.
If this current switch is explained in terms of charging, for example, it is a circuit that supplies for a fixed period of time current from a current source comprising a p-type MOS transistor P.sub.1 to the capacitance of a load circuit C.sub.O by closing switch SW1. As for the transistor that configures the current source P.sub.1, its source terminal is connected to the power source V.sub.DD, a constant voltage Vg.sub.1 is applied to its gate, sufficient voltage is applied between its gate and source, and it conducts in the saturation region. Current I.sub.1 is thus a fixed value. Switch SW1 is turned ON or OFF depending on a switching voltage V.sub.SW1.
FIG. 12 is a waveform diagram for explaining the operation of the circuit depicted in FIG. 11. When the current switch is used to charge capacitance C.sub.O, it maintains switching voltage V.sub.SW1 at a high level for a fixed period of time t1, charges capacitance C.sub.O by supplying current I.sub.1 to it, and raises the voltage of the output voltage V.sub.O. The instant switch SW1 turns ON, drain voltage V.sub.1 of the transistor P.sub.1 changes to the voltage Vn determined as a result of capacitive coupling of capacitance C.sub.O and parasitic capacitance C.sub.P1 from the level of the power V.sub.DD up until that time. Similarly, output V.sub.O also rises from the level it was at up until this time to the Vn voltage. The current I.sub.1, as shown in FIG. 12, achieves a momentary large current value at this time.
The above-described voltage Vn can be determined from the capacitance C.sub.O and parasitic capacitance C.sub.P1 circuits on either side via switch SW1. That is, if we make the capacitance C.sub.O voltage prior to switch SW1 closing equal to V.sub.O1, then, according to the law of conservation of charge, the following equation holds true. EQU C.sub.P1 V.sub.DD +C.sub.O V.sub.O1 =Vn(C.sub.P1 +C.sub.O)
Then, EQU Vn=(C.sub.P1 V.sub.DD +C.sub.O V.sub.O1)/(C.sub.P1 +C.sub.O)
In other words, if we make V.sub.O1 =0v, then the voltage value of the Vn voltage depends on the capacitance ratio of the power voltage V.sub.DD.
When current I.sub.1 is supplied to the capacitance C.sub.O for a fixed period of time in this state, the voltage V.sub.O rises. And since voltage V1 is at the same level as voltage V.sub.O, it rises in the same manner. And when V.sub.SW1 changes from a high to a low level, and switch SW1 turns OFF, then voltage V.sub.O only declines slightly, to the extent of the capacitive division of parasitic capacitance C.sub.P3 and capacitance C.sub.O within the voltage fluctuation .DELTA.V of V.sub.SW1. The result of this, as shown in FIG. 12, is that current I.sub.1 becomes an equivalent momentary negative current.
When the above-described transient current at the instant switch SW1 closes is not generated, and the transient current at the instant switch SW1 opens is not generated, the voltage V.sub.O changes as indicated in the figure by the broken line. That is, it becomes an ideal waveform. Therefore, the value of the actual voltage V.sub.O indicated by the solid line deviates from the ideal value indicated by the broken line.
With regard to the current switch discharge operation shown in FIG. 11, similarly, as a result of the current I2 becoming a transient current when switch SW2 closes, and a negative transient current when switch SW2 opens, the voltage V.sub.O deviates from the ideal value broken line.